"span" may need to be optional

[ljharb]

Although currently I believe every implementation always provides a span, @michaelficarra has raised the concern that builtins may want the option in the future to omit position information, particularly as it relates to implementation hiding.

It seems reasonable to make the span optional in preparation for this, so that users working with the stack frame structure won't rely on its presence.

[ljharb]

cc @erights @domenic

[domenic]

https://jsbin.com/celomim/edit?html,console,output in Chrome seems to indicate no span (e.g. the at Number (<anonymous>) line), although I didn't spend too much time figuring out exactly what span is.

[ljharb]

Indeed you're right; this is what I see in Safari:

"valueOf@https://null.jsbin.com/runner:6:32
Number@[native code]
map@[native code]
global code@https://null.jsbin.com/runner:9:6
write@[native code]
https://static.jsbin.com/js/prod/runner-4.1.7.min.js:1:13929
https://static.jsbin.com/js/prod/runner-4.1.7.min.js:1:10867"

and in Chrome:

"Error: check my stack
    at Object.valueOf (<anonymous>:6:23)
    at Number (<anonymous>)
    at Array.map (<anonymous>)
    at <anonymous>:9:3
    at https://static.jsbin.com/js/prod/runner-4.1.7.min.js:1:13924
    at https://static.jsbin.com/js/prod/runner-4.1.7.min.js:1:10866"

so both of these omit the Span; it'll thus have to be optional regardless.